I prefer an angled spotting scope because it seems easy to use in any position.
When shooting prone- I can set the scope just to my left and not interfere with my shooting
position. the scope is angled so that all I have to do is move my head a little to the left to look
through it and never lose the position.
Shooting off the bench ether straight or angled work fine but again I like the angled spotter
placed to my right out of the way.
For spotting for another shooter for long distances I like to set up behind and just above so I
can see the vapor trail/shock wave and the angled head makes it possible to sit down and get
a good steady position so I don't move the scope.
The scope that I use has a fully adjustable head (Will go from straight to 90o) and is very handy
in any situation or position.
The fixed angle type scope would not be an advantage over the straight one IMO.
I have a angled Kowa 821 and love it. With a tilt of the head and I can see the direction and tilt down and see through the scope. It is great for when I'm shooting prone in matches and reading the mirage.